It is already known to react low molecular weight organic polyisocyanates with low molecular weight organic polyamines to produce both ureas and biurets but, owing to the extremely high reactivity of isocyanate groups towards primary amino groups, the reaction from the said low molecular weight starting materials has not become established in large scale industrial processes, apart from a few exceptions. The main reason for this is that, in reactions carried out on a large scale, the extremely high reactivity of the aforesaid groups makes it virtually impossible to control the reaction to produce clearly defined end products.
The production of biuret polyisocyanates on an industrial scale has, therefore, hitherto preferably been carried out by the reaction of organic diisocyanates with so-called "biuretizing agents", i.e. compounds such as water, for example, which first react with isocyanate groups to form amino groups, this initial reaction then being followed by the biuretization reaction between the amine which has been formed in situ and the excess isocyanate as described in German Pat. No. 1,101,394 and U.S. Pat. No. 3,201,372. Since amino groups are never present in significant concentrations in this process, the undesirable side reactions due to the high reactivity do not occur.
Another method of overcoming the difficulties due to the high reactivity has been described in British Pat. No. 1,263,609. In this case, the diisocyanates are not reacted with free diamines but instead, the concentration of the highly reactive amino groups is reduced by the addition of carbonyl compounds to the amines.
The direct reaction between low molecular weight diprimary diamines and low molecular weight diisocyanates to produce the corresponding biuret polyisocyanates has been described in German Offenlegungsschrift No. 2,261,065 and U.S. Pat. No. 3,903,126. When the examples given in this publication were repeated, however, it was found that only the particular diamines described as preferred were suitable for the preparation of commercially usable biuret polyisocyanates, whereas the most important aliphatic diamine, hexamethylene diamine, could not be converted into a light colored biuret polyisocyanate free from sedimentation by the process according to German Offenlegungsschrift No. 2,261,065 and U.S. Pat. No. 3,903,126, in particular when it was used in combination with hexamethylene diisocyanate.
Although the process according to German Offenlegungsschrift No. 2,609,995 and U.S. Pat. No. 4,147,714, makes it possible for such light colored, sedimentation-free, biuret polyisocyanates to be prepared by the direct reaction of hexamethylene diamine with hexamethylene diisocyanate, the process described in this publication has the disadvantage that the diamine must be introduced in the gaseous state into the diisocyanate, which complicates the procedure.
The reaction between organic polyisocyanates, in particular diisocyanates, with diprimary organic diamines to produce the corresponding urea isocyanates has not in any way become established as an industrial process. In the above mentioned publications, the formation of such urea isocyanates is merely mentioned as an undesirable side effect of the preparation of biuret polyisocyanates, and no one has yet succeeded in finding a technically feasible method of utilizing the direct reaction between diprimary organic diamines and excess quantities of low molecular weight organic diisocyanates to produce the corresponding polyisocyanates containing urea groups.
The process according to the present invention described below for the first time discloses a method for reacting any organic compound containing at least two primary amino groups directly with any organic polyisocyanate to form the corresponding polyisocyanate containing urea groups or biuret groups without having to use special diamines or special auxiliary agents, for example ketones, or having to introduce the diamine in a gaseous form.
In the process according to the invention described below, the reaction can be simply controlled by suitable choice of the reaction temperature to produce either solutions of the corresponding biuret polyisocyanates in excess polyisocyanate or sedimentation-resistant, dispersions of the corresponding urea polyisocyanates in excess polyisocyanate, as desired.
The process according to the invention therefore not only provides a very simple means of preparing known biuret polyisocyanates such as those based on hexamethylene diamine and hexamethylene diisocyanate, for example, but also for the first time provides the possibility of preparing commercially, highly interesting, dispersions of urea diisocyanates in excess diisocyanate. Sedimentation resistant dispersions of this type have not hitherto been known. They constitute particularly interesting starting materials for the polyurethane chemist.